Submitted to: Ecological Applications
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/14/2007
Publication Date: 2/12/2009
Citation: Garren, J.M., Strauss, S.Y. 2009. Population-level compensation by an invasive thistle thwarts biological control from seed predators. Ecological Applications. 19:709-721. Interpretive Summary: In the reported experiments we asked the question - are the biological control agents of yellow starthistle causing enough damage to minimize the presence of or reduce populations? In short, the answer was no. We found that although the agents were greatly reducing the number of seeds produced by the plant by over 80%, on average there were still enough seeds produced to maintain yellow starthistle populations. In our experiments, we compared small populations with and without agents. We removed agents by applying insecticide to some of the small populations or plots. The insecticide successfully reduced oviposition and larval development of the insect agents inside the flower heads. In addition to the herbivore exclusion experiment, we also conducted a seed addition experiment to supplement our results. In this experiment we added YST seeds to already known populations to see if the density or seed production increased. After treatment application (on flowering adults) we recorded seed production, seedling density (during the next season), adult density, and seed production per plot. Our results were similar in both experiments. We found that more seeds were produced in plots without biocontrol agents and plots with more seeds were added; plus there were more seedlings the following fall. However, this difference in seedling density was not seen when we compared adult density. Therefore, adult density is equivalent regardless of seed input. These findings suggest that biological control efforts should be focused on inflicting adult mortality and that the current agents may only aid in controlling yst when populations are already low.
Technical Abstract: Predispersal seed predators are often chosen as biocontrol agents because of their high impacts on plant fitness; however, they have a mixed record in realizing decreased plant population growth. Few studies have experimentally removed agents to explore their impact on weed population growth. Here, we used manipulative experiments with invasive yellow starthistle (YST), Centaurea solstialis, and its pre-dispersal seed predator biological control agents, primarily Eustenopus villosus, the hairy weevil, and Chaetorellia succinea, the false peacock fly, to explore how these agents affect population growth of YST. We also use additional seed augmentation experiments to mimic effects of agents on seed inputs across a range of seed and adult plant densities. We found that biocontrol agents reduced seed production by more than 70% and that seedling numbers were significantly related to seed inputs. However, several compensatory processes prevented effective population reduction of YST by seed predators. First, self-thinning reduced seedling numbers such that densities of plants in our agents-present and agents-absent treatments converged. Second, plots in which plants started at low density had particularly high population growth rates. In this case, plant plasticity and conservation of final yield, in which a small number of large plants produce as much seed as a large number of smaller plants occupying the same area, also provided avenues where plant populations can compensate for damage. Similarly, seed production on a per plot basis was unchanged across a large range of YST densities. Our results suggest that at very low plant densities, biocontrol agents may reduce plant populations; however, other sources of mortality to YST (preferably imposed after self-thinning) will be needed to reduce populations to sizes where agents can become effective tools in weed control.